Severe Weather

 

 

Perhaps the most awe-inspiring sight in nature is the ominous, black funnel of a strong tornado. By definition, a tornado is a rapidly rotating column of air in contact with the ground and pendant from a thunderstorm cloud. The thunderous roar associated with a tornado, often described like the sound of a freight train, can be heard from miles away and may be the only warning received by those caught helpless in its path. That distinct roar echoed throughout southern Maryland on April 28, 2002. On that day, one of the strongest tornadoes ever to hit the Washington area was spawned by a fast-moving supercell that tracked for over 200 miles, from West Virginia to near Ocean City, Maryland. A supercell is an intense, long-lived thunderstorm that possesses strong rotation. (Supercell thunderstorms are explained in more detail later in this chapter.)

 

An Atmosphere Ripe for Tornadoes

 

April 28, 2002, was a textbook setup for severe weather. The Washington area was entrenched in a very warm, humid air mass ahead of a strong low-pressure center. In addition, a vigorous upper-level trough associated with cold air and a belt of strong winds in the upper atmosphere was forecast to pinwheel across the Middle Atlantic region. This was expected to trigger an eruption of powerful thunderstorms, with the potential for supercells. Supercells often cause dangerous weather in the form of high winds and large hail, and occasionally produce intense tornadoes that track for many miles. Such was the case on this particular afternoon. The first tornado, rated an F2 on the Fujita Scale, touched down near Interstate 81 in Shenandoah County shortly before 5 p.m. and destroyed or damaged nearly 100 homes, farm buildings, and businesses. Although the supercell exhibited strong rotation as it sped across the northern Virginia countryside, the only reports of severe weather were in the form of golfball-size hail and high wind gusts that downed some trees and power lines.

As the severe thunderstorm approached the Potomac River, it encountered low-level southeasterly winds funneling up the Potomac. Prior to this, winds at the surface were blowing out of the southwest. It is believed this sudden shift in wind direction helped to increase rotation in the storm that led to formation of a second, much stronger tornado. This tornado, which touched down on the Maryland side of the Potomac River in Charles County around 7 p.m., blazed a 68-mile trail of destruction. Residents who were away from their television sets or simply unaware of the impending storm, had only seconds to react to the fast-moving twister that sped along at up to 55 mph.

The events that played out on this late April afternoon were eerily similar to those witnessed by thousands of awestruck commuters at the height of rush hour on September 24, 2001. On hat day, two supercell thunderstorms produced 5 tornadoes in Virginia and in Maryland. The fifth and final tornado touched down in West Hyattsville shortly after 5 p.m. This tornadic whirl reached its peak intensity with winds close to 200 mph as it plowed through the University of Maryland campus in College Park. Tragically, two female students were killed when the car they were in was picked up and carried several hundred yards over an eight-story dormitory before landing in a clump of trees. This deadly tornado cut a 17-mile swath of destruction through west Hyattsville, College Park, Beltsville, and west Laurel before dissipating near Savage. Debris from the university campus was found up to 60 miles away in Harford County, northeast of Baltimore.

 

 

“Tornado Alley East?”

 

The 1990’s and early 21^st century have featured a rash of tornadoes in Maryland and Virginia. In fact, the annual tornado record for the greater Baltimore/Washington area has been broken three times in this time period. The busiest year in Maryland history occurred in 1995, when 24 tornadoes were reported. These numbers dwarf statistics compiled from 1950-2000 that show an average of 7 tornadoes hit Virginia and 5 tornadoes hit Maryland each year. While alarming, the upswing is probably not an indication that “Tornado Alley” is migrating eastward. First, weather events tend to display a cyclical nature. For instance, the 1920’s were very busy tornado years around the Nation’s Capital, and the 1950s and early 1960s saw a dramatic increase in tropical storms and hurricanes along the East Coast. Second, today’s sophisticated doppler radar technology has enabled meteorologists to spot tornadoes not visible in the early days. Finally, urban sprawl may also account for the increase in twister reports – development of previously uninhabited areas has resulted in more eyewitness accounts than ever before.

 

 

Mini Tornado Alleys around Washington

 

A county-by-county breakdown of tornado activity in the greater Washington area reveals something interesting. There appears to be a higher incidence of twisters in a corridor running east of the Blue Ridge through Loudoun and Frederick counties, and another “mini” tornado alley that stretches from St. Mary’s County northward into Anne Arundel County along the western side of the Chesapeake Bay.

These two areas tend to be favored locations for low-level temperature and moisture boundaries, such as bay breeze fronts that propagate inland from the Chesapeake Bay, and lee-side troughs (wind-shift lines) that form on the east side of the Blue Ridge. These low-level boundaries provide a conducive environment in which air can spin and rise, and can help spawn tornadoes as they interact with approaching thunderstorms. The good news is that the overwhelming majority of these twisters are relatively weak and stay on the ground for just a few minutes.

However, severe weather in the form of destructive winds, large hail, and deadly lightning hits Maryland, northern Virginia, and the District of Columbia numerous times each year, par-ticularly from April through July. One of the more destructive storms in recent memory occurred on April 23, 1999. Hail as large as softballs pummeled the landscape as a severe thunderstorm raced from the West Virginia panhandle southeast across northern Virginia. The storm smashed windows and windshields, dented cars and siding on homes, and shredded plants and trees. In total, the storm caused over $275 million in damage.